(1) Field of the invention
The present invention relates generally to a system and method for controlling ignition timing for an internal combustion engine. The present invention particularly relates to the system and method described above applicable to a so-called, lean burn engine, i.e., an air/fuel mixture having a lean air/fuel mixture ratio and having any kind of alcohol, blended with gasoline as its fuel is ignited and burned.
(2) Background of the art
Various types of ignition timing control systems have been proposed for internal combustion engines using gasoline as fuel thereof.
Previously proposed ignition timing control systems have generally been divided into two categories: one category being such that the ignition timing is controlled (MBT control: Minimum Advance For Best Torque control) so that a crank angular position (so called, .theta..sub.pmax) at which inner cylinder pressure becomes maximum corresponds to an angular range, generally of the compression stoke, in which engine torque generation is maximum, usually ATDC 10.degree. to ATDC 20.degree. (ATDC=After Top Dead Center); the other category being such that the ignition timing is controlled (knock avoidance control) to move toward a retardation angle to prevent recurrence of engine knocking when a knock signal produced from the engine on the basis of an inner cylinder pressure indicative signal has a level exceeding a predetermined value.
In addition, in another previously proposed ignition timing control system, combination of both MBT and knock avoidance controls has beem adopted as well as a specially designed MBT control for a multi-cylinder engine.
The latter is disclosed in a Japanese Patent Application First Publication Showa 61-14479 published on Jan. 22, 1986.
In the previously proposed ignition timing control system disclosed in the above-identified Japanese Patent Application First Publication, the ignition timing control is started in accordance with a pressure (inner cylinder pressure) signal derived from any one of the engine cylinders in which an anti-knock characteristic is relatively high and when MBT control becomes impossible due to the generation of the engine knock in the cylinder described above, the pressure signal derived from any other cylinder in which the anti-knock characteristic is also relatively high is alternatively selected. Since such a technique as described above achieves an assured MBT control in any cylinder in which no knock occurs, the rate of fuel consumption is reduced and control is simplified, as compared with a case where adoption of an average value of the crank angular position .theta..sub.pmax (the average value is a value which is deviated slightly from the real MBT).
In the case of adoption of the average value, the average value is calculated for four cylinders from the most recent .theta..sub.pmax sequentially detected for each cylinder and the ignition timing for each cylinder is calculated on the basis of the average value. Therefore, if knocking has occurred in one of the cylinders the ignition timing for the other cylinders in which no knocking occurred is unnecessarily retarded.
Since, however, the ignition timing control systems described above are applied to an engine in which gasoline (conventional fuel) is used as fuel, the ignition timing would greatly deviate from an optimum ignition timing due to a difference in fuel characteristics present, such as when gasoline is mixed with an alcohol (alternative fuel).
Hereinafter, an engine employing a blend of conventional and alternative fuels will be referred to as a mixed fuel engine.
Even in the previously proposed ignition timing control systems, it is possible to control the ignition timing toward an optimum ignition timing for alcohol or alcohol mixed gasoline using ignition timing characteristics for the gasoline provided that the engine operating conditions are in a steady state. Although, when alcohol mixed gasoline is used, deviations in the ignition timing characteristics occur corresponding to a fuel-alcohol mixture ratio, such deviations would soon be eliminated due to use of a feedback control system in the construction of the ignition timing control system.
However, the engine driving situation becomes different in the case of transient engine driving conditions. This will be explained in the detailed description.
For example, suppose a situation where an alcohol (alchohol content is 100%) only is used as fuel, the optimum ignition timing angle characteristic applied to the gasoline fuel being stored in a memory of the system as a basic ignition timing advance angle.
When the engine operating condition falls in a high load state, the basic ignition timing advance angle in the case of the gasoline fuel is largely retarded from that in the case of the alcohol fuel by 5.degree. to 10.degree..
Hence, immediately after an accelerator pedal is abruptly depressed from a low load position to the highest load (fully open) position (abrupt acceleration), ignition timing is controlled with the ignition timing angle largely retarded from an optimum position due to a response delay in the ignition timing. Therefore, the fuel is wastefully consumed and engine output is insufficiently produced. In the same way, immediately after the accelerator pedal is released from its highest load position (abrupt deceleration), the ignition timing control is, in turn, carried out with the ignition timing angle largely retarded from the optimum position due to the response delay. Therefore, ar excessive advance angle of the ignition timing occurs, and consequent engine knocking could occur.
As described above, in the case of the engine transient operating conditions, it becomes impossibe to utilize the basic ignition angle advance angle applied to the gasoline fuel for the optimum ignition advance angle applied to the alcohol or alcohol mixed fuel at the time of the engine transient operating conditions due to the response delay described above.
Another ignition timing control system has been proposed which utilizes a sensor (or program) which detects (monitors) fuel characteristics. The other ignition timing control system deals with the response delay, switching basic ignition timing advance angle maps according to detected fuel characteristics. This is exemplified by a Japanese Patent Application First Publication Showa 61-85578 Published on May 1, 1986.
However, if the same technique as disclosed in the above-described JP First Publication (Showa 61-85578) is applied to alcohol mixed fuel engines, a tremendous amount of memory capacity is required. This is because as is different from the disclosed ignition timing control system which determines whether a high-octane gasoline or normal (regularly, low-octane number) gasoline is used and changes basic ignition timing characteristics according to the result of that determination, the ignition timing advance angle characteristics are largely changed in accordance with the change of alcohol concentration (content of alcohol) in the fuel. Therefore, many ignition timing advance angle characteristic maps are required to be selectively used for providing the optimum ignition timing.
In addition, the program (or sensor) for determining the fuel characteristics described above is installed, an increase in cost of assemblying the ignition timing control system is involved. If new fuel is supplied to the engine and fuel characteristics are accordingly changed, the fuel characteristic actually injected into the engine becomes different from that during the new supply of fuel. In the same way, if an alcohol concentration sensor is installed in the engine, the cost of assemblying the ignition timing control system is not only accordingly increased but also the alcohol concentration of the fuel actually injected into the engine becomes different from that detected by the sensor. Consequently, during the transient engine operating conditions, the optimum ignition timing cannot be achieved for the alcohol mixed fuel.
Hence, it is desired to provide an ignition timing control system for the alcohol mixed fuel which always determines the alcohol concentration with high accuracy and provides the ignition timing advance angle not largely deviated from the optimum ignition timing during the transient engine driving conditions with no new addition of sensors.
To meet this demand, such an ignition timing control system as disclosed n a Japanese Utility Model Registration Application First Publication Heisei 1-159175 published on Nov. 2, 1989 has been proposed.
The detailed explanation of the disclsoed ignition timing control system will be made later.
In the ignition timing control system disclosed in the above-identified Japanese Utility Model Registration Application First Publication, the alcohol concentration is always estimated even at the time of transient operating conditions (,i.e., during the engine acceleration).
Therefore, in a case where, e.g., the ignition timing control system is applied to a so-called, lean burn engine, in which fuel combustion is carried out at a considerably leaner air-fuel mixture ratio deviated from a stoichiometric air-fuel mixture ratio, mis-firing will probably occur.
If mis-firing occurs, the following disadvantages are also likely to occur, indicating further improvements are needed. That is to say, if mis-firing occurs, the combustion state becomes an air cycle (a high amount of air and an extremely small amount of fuel) and the value of .theta..sub.pmax is fixed to TDC in the compression stroke. Therefore, the timing Total ADVance (TADV=DADV/N.times.C, C: constant, N denotes the number of engine revolutions per minute (RPM), and DADV denotes .theta..sub.pmax -STADV, STADV being the final ignition timing angle set for combustion when .theta..sub.pmax is detected.) becomes shortened. Since TADV can be deemed to represent a quantity of a combustion duration from a time at which the fuel is burnt to a time at which the combustion of fuel is generally ended and the shortened TADV can mean that the concentration of alcohol R becomes high. In the above proposed system, during transient driving conditions the alcohol concentration R is assumed to be high even if a fairly low alcohol concentration is actually present since the shortened TADV means the high concentration of alcohol, this means the ignition timing is not controlled to an optimum value and engine driveabilty and exhaust emission characteristics become worsened.